Professor Heather Jarman
Professor Heather Jarman

By Professor Heather Jarman, Research Director Nursing, Midwifery and Allied Health Professions, and clinical research lead for the Emergency Department at St George's University Hospitals NHS Foundation Trust, London

The most frequent signs and symptoms of carbon monoxide (CO) exposure are well documented in both public and health related literature as headache, flu-like symptoms, chest pain and muscle-aches. Whilst these occur in CO exposure, they are much more likely to be caused by other conditions, in migraine, or viral illness for example.

It is well-recognised that low-level CO exposure is likely to be under-diagnosed by health care professionals due to the challenges in its diagnosis and symptoms being attributed to other causes. To confirm a diagnosis of CO exposure then patients’ need to have symptoms that are suggestive of CO exposure, a raised COHb level and a confirmed cause of CO exposure.

Clinicians in emergency departments rely primarily on two factors when making a diagnosis – why the patient is in ED (the symptoms, when they started, what they are like, what makes them better or worse) and the results of any confirmatory tests they carry out (examination, blood tests, x-rays). What they find is then combined with professional judgement and the likelihood that a patient might have a condition - how commonly patients presenting with a particular symptom will have a particular disease. Think about that in the context of making a diagnosis of low-level CO exposure.

Medical equipment

Patients presenting to the ED with low-level CO exposure often do not realise they have been exposed and therefore neither the patient nor clinician raise this as a concern. If you add to this the challenges in confirming a diagnosis of CO exposure using the ‘gold standard’ measurement of carboxyhaemoglobin (COHb), due to the levels of COHb in the blood dropping rapidly before a measurement is taken, then the CO diagnosis becomes even more unlikely. For instance, a person who is exposed to CO in the home (causing a rise in blood CO levels) may have a normal COHb level once this is taken in the hospital.

Symptoms of CO poisoning
Symptoms of CO poisoning

Finally, data from NHS Digital shows there were 17.4 million attendances to Emergency Departments in the period from April 2020 to March 20211, of these literature reports around 4000 ED attendances with CO exposure annually, equating to approximately 0.02% of all attendances. For ED clinicians this is a rare condition.

The research project:

The carbon monoxide exposure project was led by myself, Professor Heather Jarman with the aim of establishing the prevalence of low-level CO exposure in patients presenting to the ED.

The study:

Patients aged 18 or over attending the emergency departments of four UK hospitals with symptoms of low-level CO exposure between December 2018 and March 2020 were screened for possible CO exposure. Patients with cardiac chest pain, non-traumatic headache, flu-like symptoms, seizures and syncope were consented to participate.

Those with known smoke inhalation from fire-related incidents were excluded. Data on symptoms were collected along with a COHb taken as soon as possible after arrival in the ED. We used a screening tool ‘COMA’ to ask patients about CO exposure risk. This tool was devised by Public Health England (now the UK Health Security Agency) and has the following questions:

  • Companions (Is anyone else at home affected by the same symptoms?)
  • Outside (Does the symptoms disappear when outside your house?)
  • Maintenance (Are your fuel burning appliances regularly checked?)
  • Alarm (Do you have a carbon monoxide alarm??

If the patient answers yes to either of the first 2 questions or no to either of the last 2 questions, then the clinician is prompted to consider CO exposure and take appropriate clinical and public health measures.

Patients who were found to have an abnormally high COHb level or where there was a strong clinical suspicion of CO exposure from the COMA screening questions, were asked to contact an emergency gas engineer, as is normal practice, to investigate any potential source of exposure at home or work.

We then aimed to use any data that was available from the gas engineers to confirm an source of CO (raised CO in the home) or probable cause (faulty gas appliance found).

The results:

We analysed data from 4175 patients. 26 (0.62%) had suspected CO exposure based on COHb level and/or COMA questions. Data linked to CO testing in the home confirmed 1 case of CO presence and 21 probable cases based on a possible CO source from gas appliances. The majority of probable cases had normal COHb level in ED and were identified using only the responses to the COMA screening tool. This further supports the challenges of using COHb to identify patients with low level CO exposure and the need for clinicians to be alert to the possibility of CO exposure.

Professor Jarman will be talking in more detail about her project for our March Lecture, on Thursday 17th March at 3pm - 4pm. To sign up to this presentation please click here.


1 Hospital Accident & Emergency Activity 2020-21 - NHS Digital

2 Diagnosing Poisoning: Carbon Monoxide (CO) (